GreaserPull on your tightest jeans and a white T-shirt and roll up a deck of cards or pack of cigs in one sleeve.

Gang-BangerBaggy jeans, solid-colored XXL T-shirt, possibly a bandanna and chains. Works especially well if you are a Hasidic Jew. Just avoid the real local gang colors and walking around the East Side and you should be fine.

Rehab-Hopping CelebrityDress in wrinkled clothes you wore four days ago. Carry a half-empty bottle of Jager or whatever you can find in your alkie parent's/uncle's/WoW buddy's stash. Slur your words while whining about your unappreciated talent. When asked to demonstrate your talent, sing (badly), dance (shuffle, stumble, and fall over), or recite the Pledge of Allegiance with random words thrown in. LADIES: to make an unmistakable first impression with your costume, eschew panties and spread knees while tumbling out of car.

No costume required:

VegetarianYou don't even have to turn down the pork ribs! After all, people dressed as zombies don't actually eat brains.

RepublicanWhen anyone asks what you are, just say "Thank the Good Lord for great leaders like Sarah Palin!" (That would certainly scare me.)

Person Afflicted with an Unfortunate Case of Tourette'sThis is one character that gets easier and easier the drunker you get. CAUTION: This costume idea is not PC.

Insane Fanboy/FangirlGet your geeky younger sib to provide some classic quotes from any sci-fi TV show or movie, video game, or anime/manga series. For example, pontificate loudly on the pros and cons of the various battle systems of the Final Fantasy games. (I am disqualified for this one.)